Chapter 27 / The Vestibular System 631
Figure 27–2 Hair cells in the vestibular
labyrinth transduce mechanical stimuli
into neural signals.At the apex of each cell
are the stereocilia, which increase in length
toward the single kinocilium. The membrane
potential of the receptor cell depends on the
direction in which the stereocilia are bent.
Deflection toward the kinocilium causes the
cell to depolarize and thus increases the rate
of firing in the afferent fiber. Bending away
from the kinocilium causes the cell to hyper-
polarize, thus decreasing the afferent firing
rate. (Adapted, with permission, from Flock
1965.)
receive convergent signals from other systems such as
proprioceptors, visual signals, and motor commands.
Central processing of these multimodal signals occurs
very rapidly to ensure adequate coordination of visual
gaze and postural responses, autonomic responses,
and awareness of spatial orientation.
The Vestibular Labyrinth in the Inner Ear
Contains Five Receptor Organs
The membranous labyrinth is supported within the bony
labyrinth by a filamentous network of connective tissue.
The vestibular portion of the membranous labyrinth lies
lateral and posterior to the cochlea. Vestibular receptors
are contained in specialized enlarged regions of the mem-
branous labyrinth, termed the ampullae for the semicir-
cular canals and maculae for the otolith organs (Figure
27–1B). Both of the otolith organs lie in a central compart-
ment of the membranous labyrinth, the vestibule, which
is surrounded by the bony labyrinth of the same name.
The membranous labyrinth is filled with endo-
lymph, a K
+
-rich (150 mM) and Na
+
-poor (16 mM) fluid
whose composition is maintained by the action of ion
pumps in specialized cells. Endolymph bathes the sur-
face of the vestibular receptor cells. Surrounding the
membranous labyrinth, in the space between the mem-
branous labyrinth and the wall of the bony labyrinth,
is perilymph. Perilymph is a high-Na
+
(150 mM), low-K
+
(7 mM) fluid similar in composition to cerebrospinal
fluid, with which it is in communication through the
cochlear duct. Perilymph bathes the basal surface of
the receptor epithelia and the vestibular nerve fibers.
Two fluid-tight partitions in the bony labyrinth, the
oval and round windows (Figure 27–1B), connect the
perilymphatic space to the middle ear cavity. The oval
window is connected to the tympanic membrane by
the middle ear ossicles. These windows are important
for sound transduction (Chapter 26). The endolymph
and perilymph are kept separate by a junctional com-
plex of support cells that surrounds the apex of each
receptor cell. Disruption of the balance between these
two fluids (by trauma or disease) can result in vestibu-
lar dysfunction, leading to dizziness, vertigo, and spa-
tial disorientation.
During development, the labyrinth progresses
from a simple sac to a complex set of interconnected
sensory organs, but retains the same fundamental top-
ological organization. Each organ originates as an epi-
thelium-lined pouch that buds from the otic cyst, and
the endolymphatic spaces within the several organs
remain continuous in the adult. The endolymphatic
spaces of the vestibular labyrinth are also connected
to the cochlear duct through the ductus reuniens
(Figure 27–1B). In addition, the membranous laby-
rinth contains a small tube, the endolymphatic duct,
which extends through a space in the sigmoid bone,
the vestibular aqueduct, to end in a blind sac adjacent
to the dura in the epidural space of the posterior cra-
nial fossa. It is thought that the endolymphatic sac has
both absorptive and excretive functions to maintain
the ionic composition of the endolymphatic fluid.
Hair Cells Transduce Acceleration Stimuli Into
Receptor Potentials
Each of the five receptor organs has a cluster of hair
cells responsible for transducing head motion into ves-
tibular signals. Hair cells are so named due to an array
of nearly 100 staggered height stereocilia. The shortest
stereocilia are at one end of the cell and the tallest at
the other, ending with the only true cilium of the hair
cell, termed the kinocilium. The kinocilium is typically
the tallest of all stereocilia. Angular or linear accelera-
tion of the head leads to a deflection of the stereocilia,
which together compose the hair bundle (Figure 27–2).
受体电位 去极化
超极化
神经冲动 静息放电
脉冲频率
增加
脉冲频率
降低
兴奋 抑制
Kandel-Ch27_0629-0650.indd 631 09/12/20 5:01 PM